This invention relates to carbon artefacts having anti-oxidation coatings over at least part of their surfaces and has one application in carbon brake discs such as are used in aircraft disc-brake assemblies. "Carbon" in this Specification includes graphite.
An aircraft disc-brake assembly may, for example, comprise a plurality of rotor discs keyed at their outer edges to the wheel hub, and a plurality of stator discs keyed at their inner edges to a non-rotating torque tube, the two sets of discs being interleaved. To operate the brake, axial pressure is applied to the stack of interleaved plates, braking being obtained by the rubbing friction between the adjacent flat surfaces of the rotor and stator discs. Such an assembly is described, for example, in "Aircraft Engineering", vol 43, pp 12-14 (June 1971). The discs may be made of steel, but the alternative use of carbon-carbon composite discs has known advantages. For example such discs are lighter than steel discs and are less liable to seize-up from overheating, eg in an emergency stop.
With carbon discs, however, the high temperatures attained during braking tend to cause oxidation of those areas of the disc surface exposed to the atmosphere, in particular the inner and outer peripheral regions; the rubbing surfaces themselves are mutually shielded from the atmosphere when in contact. Such oxidation is, of course, undesirable.
It has already been proposed to apply protective coatings to such areas to prevent or reduce oxidation. For example UK Patent Specification No. 1,311,537 discloses coatings of several alternative substances, including inter alia silicon, nickel and chromium, and German Offenlegungsschrift 2,306,631 (UK Patent Specification No. 1,415,853) describes the use of a mixture of boron-containing and carbonisable organic materials.
The present invention provides a protective coating comprising a combination of superimposed layers of specified substances which has been found particularly effective.